Silver stain following SID-PAGE of our approximately 250,000 X purified VIII: C preparations shows major reproducible bands at approximately 240,000-270,000 for nonreduced VIII: C and approximately 80,000 for reduced VIII:C. These M.V. s were obtained by two fundamentally different purification procedured, one based on a large scale polyelectrolyte procedure, the other on a bench scale immuniaffinity procedure. The isolation of a nearly homogeneous bank for normal VIII: C will now allow us to isolate both the 270,000 and the 80,000 M.W. materials, respectively, as well as the othe M.W. forms of VIII:C by gel exclusion chromatograph in the presence of inhibitors and/or by preparative acrylamide gel electrophoresis. This will permit us to interpret the effects of proteolysis (especially by thromb in and protein C) on procoagulant activity, molecular weight, and antigenic activity of the isolated cleaved fragments against homologous and heterologous antibodies, and determine the binding of each to vWF, their isoelectric points, amino acid and carbohydrate composition and their N-terminal amino acids. If the 270,000 material is a precursor of the 80,000 M.W. fragment, as we expect, the cleaved material maybe identified by SDS-PAGE after limited proteolysis of the 270,000 M.W. material; or it may prove to be a subunit that can be demonstrated by reduction of monoclonal antibodies. The latter can be used to isolate each by immunoaffinity chromatography with moderate or low affinity antibody and to determine the immunochemical relationships of the two proteins, and to correlate the antigenic and procoagulant activity with structure.